The present invention relates to a fuel filter for filtering foreign matter from fuel drawn by a fuel pump from a fuel tank. In particular, the present invention relates to an in-tank fuel filter system that is used in a fuel tank, and more particularly, to an in-tank fuel filter system that has improved resistance to static electricity.
In this specification, a fuel pump is simply referred to as a pump, a fuel tank as a tank, and a filter used in the tank as an in-tank filter system.
An example of such an in-tank filter system is disclosed in German Patent Application No. P 42 42 242.6 (which corresponds to Japanese Laid-Open Patent Publication No. 6-213091). In this example, a filter is constructed in modules by assembling a generally cylindrical filter unit around a generally cylindrical pump. The filter module is disposed for use within a tank.
The known in-tank filter module system is very effective for simplifying the process of mounting the pump and the filter unit in the tank. However, in this design, no particular consideration was given to the fact that the filter and the fuel become electrically charged.
When fuel passes through the filter element to filter foreign matter from the fuel, the fuel flows against the filter element. Because of friction, the fuel becomes positively charged and the filter element becomes negatively charged. When the fuel is charged, the fuel piping also becomes charged. The fuel piping is normally attached to a vehicle body by an insulating elastic element, such as a rubber bushing, in order to protect the fuel piping from vibrations. Therefore, the fuel piping is electrically insulated from the vehicle body. Electrification of the fuel piping causes an electric discharge between the fuel piping and the metal vehicle parts adjacent to the fuel piping. The electric discharge may possibly damage the fuel pipe wall. Some instances of serious fuel pipe wall damage have actually occurred as a result of repeated static electric discharges.
Further, when the filter element becomes charged, this charge builds up on the filter element and the electric potential increases. As a result, the life of the filter element may be shortened and/or spark discharges may be generated. If the filter cover forming the filter surface is made of a nonconductive material such as a resin, the possibility of generating a spark discharge is reduced. However, the resin deteriorates as a result of this electrification and thus, the filter life is shortened. If the filter cover is made of a conductive material such as a metal, the life is not shortened significantly, but spark discharges tend to be readily generated. Thus, the charge generated by the filtering process causes serious problems for the filter and the fuel piping as well.
In the known filter, however, no particular consideration was given static charge build up. For example, in the known filter, the fuel is filtered by flowing axially, instead of radially, with respect to the cylindrical filter element. The amount of static charge that is generated in the fuel or on the filter element, when the fuel passes through the filter element to filter foreign matter from the fuel, is not only influenced by the total amount of fuel passing through the filter, but also by the flow rate and the time required for the fuel to pass through the filter element. The faster the flow rate, the more readily the filter element is electrically charged. Further, the longer the time required for the fuel to pass through the filter element, the more readily the filter element is electrically charged.
If a cylindrical filter element is used to filter the fuel, when the filter element is designed such that the fuel flows axially therethrough, the fuel flow rate, the contact distance of the fuel with the filter element and the time required for the fuel to pass through the filter element are increased as compared with a filter element in which the fuel flows radially through the filter element. To the contrary, when the fuel flows radially, the fuel flow rate, the contact distance of the fuel with the filter element and the required time for the fuel to pass through the filter element are reduced. As a result, the amount of electric charge generated on the filter element, through which the fuel flows radially, is much less than an axial-flow type filter element. Nevertheless, in the known in-tank filter system, the axial fuel filtering method is utilized and no attempt was made to prevent the filter and the fuel from becoming electrically charged.
The most common method taken to prevent electrification of the filter is to discharge the electric charge. It is generally believed that the electric charge cannot be discharged if the filter cover forming the filter surface is made of resin. Therefore, in the commonly used designs, the filter cover is made of metal and a ground wire is connected between the metal cover and the vehicle body or the like, so that electric charge built up on the filter cover is discharged to the vehicle body or the like. When the filter is made of metal, the manufacturing cost is increased, and the design options for the cover configuration are limited. In view of these factors, a method in which the filter cover is made of conductive resin was proposed and disclosed in International Patent Publication No. WO 92/04097 (corresponding to Japanese Laid-Open Patent Publication No. 6-500373). This publication discloses a technique in which a filter cover is made of a conductive resin and a ground wire is connected between the cover and the vehicle body, so that electric charge built up on the filter cover is discharged.
However, some disadvantages are recognized in such a design in which the filter cover is made of a conductive material, such as a metal or a conductive resin, and is connected to a ground wire to discharge the electric charge built-up on the filter. First, the electric charge on the filter can be discharged, but the electric charge in the fuel cannot be discharged. Further, no measures are taken to prevent electrification of the fuel piping. As described above, if the fuel piping, which is normally connected to a vehicle body or the like by an insulating member such as a rubber bushing, becomes electrically charged, spark discharges will be generated between the fuel piping and the vehicle body, which may seriously damage the fuel piping. This problem is not eliminated even if the filter cover is made of a conductive material and is connected to a ground wire.
Further, as disclosed in WO92/04097, if the filter cover is made of a conductive resin, the bulk resistivity of the filter cover can not be significantly reduced, and the electrifying potential can not be reduced to zero, even if a ground wire is connected to the filter cover. It is particularly difficult to uniformly reduce bulk resistivity and some portions of the filter cover tend to have locally high resistance. As a result, some portions tend to have locally high electric potential. When the portions having high potential are brought close to the tank during replacement of the filter or a similar operation, a spark discharge may be generated. Further, concentrated electric discharge currents pass through the conductive resin in the vicinity of the mounting portion of the ground wire, so that the resin is likely to deteriorate.
If the filter cover is made of metal, most of the above problems can be solved. However, serious problems still remain unsolved, such as the filter cover manufacturing costs are increased and the filter cover design options are limited. Further, the problem that the fuel or the fuel piping will become electrically charged is not solved.
Accordingly, an object of the present invention is to teach a new in-tank filter system that can effectively cope with a troublesome problem of electric charge that is caused by the known filter.
Another object of the invention is to form a filter cover without using a special resin, such as a conductive resin.
Still another object of the invention is to eliminate the need to connect a ground wire to the filter cover.
In one aspect of the present invention, a cover of a generally cylindrical filter element that is disposed for use in a tank is made of a nonconductive resin and the filter element is of the type that filters foreign matter from the fuel by causing the fuel to flow radially therethrough. The above-noted WO92/04097 discloses the use of a filter element that filters foreign matter from fuel by causing the fuel to flow radially therethrough and to form a cover for the filter element from a nonconductive resin. However, the filter of this known design is used outside of the fuel tank. Accordingly, it was recognized in this publication that the resin filter cover might be damaged by electrification using this type of filter. Therefore, the publication proposed to form the filter cover from a conductive resin in order to prevent such occurrence.
However, the present inventors conducted various experiments and found that a nonconductive resin filter cover can be disposed within the tank and the resin cover can be prevented from rapidly deteriorating as a result of electrification of the nonconductive resin cover, if the fuel flows radially through the filter element so that the filter element does not easily become electrically charged. Thus, a long service life can be achieved. As a result, the present inventors have succeeded for the first time in making a commercially viable in-tank filter system having a nonconductive resin cover.
In the filter according to this aspect of the invention, the filter cover is not grounded to the vehicle body or the like and therefore, its electrifying potential is not zero. However, because the filter cover is made of nonconductive material having a high bulk resistivity, rapid movement of the electric charge is restrained. Thus, even if the resin filter cover is brought close to a metal part having a different electric potential, a spark discharge will not be generated.
In another aspect of the invention, in a filter that is formed by assembling a generally cylindrical filter element around an outer periphery of a generally cylindrical pump, the surface of the filter element is made of a nonconductive material, and the generally cylindrical filter element is of the type that filters foreign matter from the fuel by causing the fuel to flow radially therethrough. With this type of the filter element, the amount of electric charge can be reduced. Therefore, by covering the filter element with a nonconductive cover and disposing it within a tank, corona discharge is generated from the surface of the cover when the cover is exposed above the fuel remaining in the tank to thereby prevent electric charge problems. The energy of the corona discharge is low, so that no particular problems are caused. Thus, as a result of corona discharges, the amount of electric charge on the filter cover surface can be reduced to such an extent that the life of the resin cover is not shortened. Further, because the filter cover is made of a nonconductive material having a high bulk resistivity, rapid movement of the electric charge is restrained. Thus, even if the resin filter cover is brought close to a metal part having a different electric potential, a spark discharge will not be generated.
In a still another aspect of the invention, the filter element has two covers. In this specification, the inner cover will be called a case and the outer cover will be called a housing. Both the case and the housing cover the filter element. At least the outer cover (housing) is made of a nonconductive material and the filter is disposed within a tank. With such a construction, electric charge generated on the filter surface can be reduced to such an extent that the housing life is not shortened.
In a further different aspect of the present invention, return fuel flows along the filter. Thus, electric charge generated on the filter is discharged and sounds produced by the return fuel dropping into the fuel tank are reduced.
The present invention will be more apparent from the following detailed description of the best modes for performing the invention.